A MZ (Mach-Zehnder)-type optical modulator is configured to branch the light having entered from the input-side optical waveguide into two branched lights in two optical waveguides (arms) with 1:1 intensity. The branched lights are allowed to propagate over a fixed length, and subsequently multiplexed again, thereby outputting the resultant light. Phase modulation units provided in the two branched optical waveguides can be used to change the phases of the two lights to thereby change the light interference conditions when the lights are multiplexed. Thus, the output light can have a modulated intensity or phase.
The optical waveguide of the phase modulation unit is made of material such as dielectric substance (e.g., LiNbO3) or semiconductor (e.g., InP, GaAs, Si). A modulation electric signal is inputted to an electrode provided in the vicinity of these optical waveguides to apply a voltage to the optical waveguides, thereby causing a change of the phases of lights propagating along the optical waveguides.
A principle to cause a change of the light phase is mainly provided by the Pockels effect in the case of LiNbO3 or the Pockels effect or the Quantum Confined Stark Effect (QCSE) in the case of InP or GaAs. Si mainly provides the carrier plasma effect.
In order to provide optical communication requiring low power consumption at a high speed, such an optical modulator is required that has a high modulation rate and that has a low driving voltage. In order to provide light modulation at a high speed of 10 Gbps or more and with an amplitude voltage of a few volts, a travelling wave electrode is required. A high-speed modulation electric signal travelling along a travelling wave electrode is matched in speed with the light propagating in the optical waveguide, thus an electric signal is allowed to interact with light while propagating. An optical modulator with a travelling wave electrode of a few millimeters to a few dozens of millimeters length has been put into a practical use (see for example NPL 1).
In the case of this travelling wave electrode type optical modulator, an electrode structure and an optical waveguide structure having a low loss and a small reflection are required that allow an electric signal or light propagating in the waveguide to propagate without causing a deteriorated light intensity.
Furthermore, one of the MZ-type optical modulator has an Si optical modulator in which an optical waveguide is made of Si. The Si optical modulator is configured on an SOI (Silicon on Insulator) substrate. SOI substrate is obtained by allowing an Si thin film to adhere on an oxide film (BOX) layer obtained by thermally-oxidizing the surface of an Si substrate. The Si optical modulator is manufactured by machining the Si thin film into a fine wire so that light can be wave-guided through the Si wire. After which dopant is injected into the Si wire so as to obtain a p-type/n-type semiconductor junction. And after, SiO2 is deposited to provide a light clad layer, then electrode formation are performed.
This process requires the optical waveguide to be designed and machined so as to achieve a low light loss. Thus, the p-type/n-type semiconductor must be doped and the electrode must be prepared so as to minimize the light loss and to minimize the reflection and loss of a high-speed electric signal.
FIG. 1 is a cross-sectional view illustrating an optical waveguide as a base of a conventional Si optical modulator. In FIG. 1, it is assumed that light propagates in a direction vertical to the paper. This Si optical modulator has an optical waveguide configured by an Si layer 2 sandwiched between upper and lower SiO2 clad layers 3 and 1. The Si layer 2 has, at the center of the drawing, an Si fine wire unit for the purpose of confining light. The Si fine wire unit has a cross-sectional structure called as a rib waveguide having a different thickness from that of the periphery.
The center of this Si layer 2 is a Si layer 201 having a thick thickness. The Si layer 201 is an optical waveguide core having a refractive index different from the SiO2 clad layers 1 and 3 at the periphery to confine light propagating in the direction vertical to the paper, thereby configuring an optical waveguide 7.
The optical waveguide 7 is interposed between slab regions 202 at both sides and has a high concentration p-type semiconductor layer 211 and a high concentration n-type semiconductor layer 214. The optical waveguide 7 has, at the center of the core, a pn junction structure consisting of an intermediate concentration p-type semiconductor layer 212 and an intermediate concentration n-type semiconductor layer 213 formed by doping. From the both left and right ends of FIG. 1, a modulation electric signal and a bias are applied.
The pn junction structure formed by the intermediate concentration p-type semiconductor layer 212 and the intermediate concentration n-type semiconductor layer 213 also may have a pin structure sandwiching an undoped i-type (intrinsic) semiconductor not shown.
The optical waveguide 7 allows light to propagate therein so as to move along this pn junction (the direction vertical to the paper). Although not shown in FIG. 1, metal electrodes connected to the high concentration semiconductor layers 211 and 214 at both ends can be provided. These metal electrodes are used to apply, to a pn junction unit, a modulation electric signal of an RF (radio frequency) and a reverse bias electric field (an electric field from the right side to the left side in FIG. 1).
By the structure as described above, the carrier density in the interior of an optical waveguide core 201 can be varied to change the refractive index of the optical waveguide (carrier plasma effect), thereby modulating the phase of light.
The size of the waveguide depends on the refractive index of material used for a core/clad, and thus cannot be determined uniquely. The rib-type silicon waveguide structure as shown in FIG. 1 that has the optical waveguide core part 201 and the slab regions 202 at both sides has, as an example of the size including a waveguide core, a width of 400 to 600 (nm)×a height of 150 to 300 (nm)×a slab thickness of 50 to 200 (nm)×and a length of about a few (mm).
These Mach-Zehnder optical modulators using the optical waveguide as described above include, as conventionally-known, two-types of structures called a single electrode type and a dual electrode type. They are classified based on the difference of the electrode structure.
These electrodes are provided along two optical waveguides constituting two arms of a Mach-Zehnder optical modulator. These electrodes consist of two RF electrodes for applying a pair of differential signal voltages for modulation and at least one fixed potential electrode for applying a fixed potential.
In the case of the single electrode type structure, one fixed potential electrode is provided between the two RF electrodes to apply a DC bias potential, and thus is called a DC electrode. In the case of the dual electrode type structure, fixed potential electrodes are provided between the two RF electrodes and at the both outer sides of the two RF electrodes to apply a 0 Volt ground potential (grounding potential), and thus are called ground (GND) electrodes.
Conventional Single Electrode Type Mach-Zehnder Modulator
FIG. 2 is a plan view illustrating a Si optical modulator of a conventional single electrode type Mach-Zehnder modulator. FIG. 3 illustrates a cross-sectional view taken at III-III of FIG. 2 (see for example NPL 2).
In the plan view of FIG. 2, the input light from the left side is branched by optical waveguides 7a and 7b. Branched lights are phase-modulated by the modulation electric signal (RF signal) applied between upper and lower RF electrodes 5a, 5b and a center DC electrode 6. Phase-modulated lights are subsequently coupled and the resultant modulated light is outputted through the right end, thereby constituting a single electrode type Mach-Zehnder modulator.
The cross-sectional view of FIG. 3 taken at III-III line in FIG. 2 shows a basic structure in which two optical waveguides having a cross-sectional structure similar to that of FIG. 1 are arranged symmetrically in the left-and-right direction.
On the clad layer 3, at both of the left and right side ends, two radio frequency lines (RF electrodes 5a and 5b) to input one pair of differential modulation electric signals (RF signals) are provided. In the center of the clad layer 3, the DC electrode 6 to apply a common bias voltage is provided.
The two RF electrodes 5a and 5b have therebetween the Si layer 2 including the two optical waveguides 7a and 7b interposing the DC electrode 6. The optical waveguides 7a and 7b have pn junction structures formed symmetrically in the left-and-right direction. The RF electrodes 5a and 5b are electrically connected to the high concentration p-type semiconductor layer 211 by way of a via 4 (penetration electrode), respectively.
The DC electrode 6 is similarly connected to the high concentration n-type semiconductor layer 214 at the center. When the DC electrode 6 is applied a positive voltage relative to the RF electrodes 5a and 5b, a reverse bias can be applied to the two left and right pn junction units. In the following section, it is assumed that these electrodes and semiconductor layers are similarly electrically connected by way of one or plurality of via(s) 4.
FIG. 4(a) illustrates a doping status of the semiconductor in the cross section III-III and FIG. 4(b) illustrates a band level diagram during light modulation.
The Si optical modulator of the single electrode type has the following merits. Specifically, because a reverse biases are applied to the pn junctions, the RF electrodes and the DC electrode are electrically independent, thus eliminating the need of actively applying bias voltages to the RF electrodes. This consequently advantageously provides a simpler configuration not requiring a bias tee circuit for applying a bias to the RF electrode, or a capacitor for DC block between a driver IC and a RF electrode for example.
In the above description, an example has been described in which the RF electrode is abutted to the p-type semiconductor layer while the DC electrode is abutted to the n-type semiconductor. However, a reverse configuration also may be used in which the RF electrode is abutted to the n-type semiconductor layer while the DC electrode is abutted to the p-type semiconductor layer. In this case, a bias voltage applied to the DC electrode can be a negative voltage relative to the RF electrode to thereby applying reverse biases to the pn junction units.
Conventional Dual Electrode Type Mach-Zehnder Modulator
FIG. 5 is a plane view illustrating a Si optical modulator of a conventional dual electrode type Mach-Zehnder modulator. FIG. 6 illustrates a partial cross-sectional view taken along VI-VI line in FIG. 5.
In the plan view of FIG. 5, the input light from the left side is branched by the optical waveguides 7a and 7b. Branched lights are phase-modulated by the modulation electric signals (RF signals) applied to the upper and lower RF electrodes 15a and 15b. Phase-modulated lights are subsequently coupled to output the resultant modulated light through the right end, thereby constituting a dual electrode type Mach-Zehnder modulator.
The partial cross-sectional view shown in FIG. 6 taken along the VI-VI line in FIG. 5. FIG. 6 illustrates a cross section of a part having the optical waveguide 7a of a cross-sectional structure similar to FIG. 1. FIG. 6 also illustrates one of radio frequency lines (RF electrode 15a) to input a differential modulation electric signal (RF signal), and ground electrodes 16a and 16c interposing the RF electrode 15a. The RF electrode 15a and the ground electrode 16a have therebetween the Si layer 2 including the optical waveguide 7a in which a pn junction structure is formed. The clad layer 3 has thereon the RF electrode 15a and the ground electrode 16a. RF electrode 15a is electrically connected, by way of the via 4, to the high concentration n-type semiconductor layer 214, and ground electrode 16a is electrically connected, by way of the via 4, to the high concentration p-type semiconductor layer 211 of the Si layer 2.
In the case of this dual electrode type modulator, the ground electrode 16c provided at the center of FIG. 5 (the right side of FIG. 6) is not directly abutted to the semiconductor layer but is used to provide a ground potential. Thus, the RF electrode 15a and the ground electrodes 16a, 16c form a radio frequency transmission line having a GSG (Ground-Signal-Ground) structure. This structure can be used to adjust the characteristic impedance of the transmission line and to improve the transmission characteristic. Furthermore, the RF signal line surrounded by the ground electrodes can be used to form such an optical modulator that causes smaller signal leakage and that causes reduced crosstalk or propagation loss.
Although not shown in FIG. 6, the optical waveguide 7b has an electrode and a semiconductor area of similar structure. As can be seen from FIG. 5, a semiconductor area corresponding to the optical waveguide 7b is formed separately from a semiconductor area corresponding to the optical waveguide 7a. These regions are arranged in a symmetrical manner interposing the center line of the center ground electrode 16c in the up-and-down direction of FIG. 5 (or in the left-and-right direction of FIG. 6), and also have symmetrically arranged doping statuses.
The Si optical modulator as a radio frequency transmission line has a characteristic impedance that is significantly influenced by the capacitance of the pn junction unit of the Si layer. In the case of the dual electrode Si modulator, the capacitance between the RF electrode 15a and the ground electrode 16c can be adjusted to thereby change the characteristic impedance in a relatively easy manner to achieve about 50 Ohm in the single end and about 100 Ohm in the differential driving.
In the case of the dual electrode type Si optical modulator as described above, DC bias voltages are to be applied in a superposed manner to the RF electrodes, thus requiring, when compared with the single electrode type Si modulator, a bias tee circuit for the connection to the driver IC. However, as described above, the control of the capacitance between the RF electrode 15a and the ground electrode 16c provides an advantage that the characteristic impedance can be changed in a relatively easy manner. The dual electrode type thereby provide, together with the existence of the surrounding ground electrode, such an optical modulator that causes a smaller signal leakage and reduced crosstalk and propagation loss.
In the above description, an example has been described in which the RF electrode is abutted to the n-type semiconductor layer while the ground electrode is abutted to the p-type semiconductor layer. However, a reverse configuration also may be used in which the RF electrode is abutted to the p-type semiconductor layer while the ground electrode is abutted to the n-type semiconductor layer. In this case, a bias voltage applied together with the RF signal to the RF electrode can be a reverse bias to the pn junction unit by applying a negative voltage relative to the ground electrode.